Post-Fire Axial Compression Performance of Square Steel Tube Concrete Members

Literature Overview

This paper by Yang Hua and Han Linhai, published in 2001 in the Journal of Harbin Institute of Technology, presents experimental and numerical investigations into the residual mechanical behavior of square steel tube concrete (SRC) short columns after exposure to constant high temperatures ranging from 20°C to 900°C. Ten specimens were tested to evaluate the load-deformation relationships following thermal exposure. The research is significant because it addresses a critical gap in structural engineering practice: how fire-exposed composite members retain their load-bearing capacity and deformation resistance, which directly informs post-fire structural assessment and rehabilitation decisions.

Core Technical Content and Experimental Methodology

The experimental program employed ten square steel tube concrete short specimens subjected to constant temperatures spanning the full range from ambient (20°C) to 900°C. After thermal exposure, each specimen was loaded axially to failure, and the complete load-deformation curve was recorded. The fundamental experimental parameters included the exposure temperature T, the geometric dimensions of the square steel tube, the concrete grade, and the steel tube grade.

The key experimental findings reveal several important engineering conclusions:

• The residual strength and elastic modulus of SRC members decrease progressively with increasing exposure temperature, which is consistent with the well-known degradation of both steel and concrete at elevated temperatures.
• Despite significant strength loss at high temperatures, SRC members retain relatively good deformation resistance and maintain a notable level of post-fire residual load-bearing capacity.
• The confinement effect of the square steel tube provides a critical advantage over plain concrete, as the steel tube continues to provide lateral restraint even after partial loss of concrete strength.

Residual Strength Reduction Model

Based on the core concrete mechanical model established at ambient temperature, the authors developed a modified constitutive model for the core concrete after constant temperature exposure. This model accounts for the temperature-dependent reduction in both compressive strength and elastic modulus of concrete. The simplified calculation formulas proposed for residual axial compression load-bearing capacity and axial compression modulus showed good agreement with experimental results, validating the numerical analysis approach.

Engineering Practice Implications

From a practical standpoint, this research has direct relevance to several engineering scenarios. First, in post-fire structural assessment, engineers can use the proposed simplified formulas to estimate the residual load-bearing capacity of SRC members without conducting destructive testing on every member. Second, the findings support the use of SRC systems in fire-prone environments, as the composite action provides inherent fire resistance that exceeds what either material alone could achieve.

The confinement effect of the square steel tube is particularly important. Unlike plain concrete columns that may spall catastrophically during fire exposure, the steel tube maintains geometric integrity and continues to provide lateral confinement, which prevents explosive spalling and maintains a residual load path. This has direct implications for the design of fire-resistant structural systems in high-rise buildings, industrial facilities, and infrastructure exposed to potential fire hazards.

For welding engineers specifically, this research highlights the importance of ensuring the integrity of welded connections in SRC members. If the steel tube is fabricated from welded pipe sections, the weld quality becomes critical for post-fire performance. Welded joints that may have been acceptable at ambient temperature could become critical weak points after thermal cycling, as the heat-affected zone (HAZ) may have different thermal expansion characteristics and residual stress distributions compared to the base metal.

Key Questions and Reflections

Several questions emerge from this research that warrant further investigation. The study focuses on constant temperature exposure, which represents a simplified thermal history. In real fire scenarios, the temperature profile is typically non-uniform and time-dependent, with rapid heating followed by potential cooling. The transition from heating to cooling can introduce additional thermal stresses that may further compromise the structural integrity of SRC members.

Additionally, the study examines only short columns under pure axial compression. In practice, SRC columns are almost always subjected to combined axial and bending loads, which would amplify the effects of post-fire degradation. The interaction between residual strength, residual stiffness, and second-order effects (P-delta effects) after fire exposure remains an area requiring further research.

Study Insights and Conclusions

This paper provides a valuable foundation for understanding the post-fire behavior of SRC members. The proposed simplified calculation formulas offer practical tools for structural engineers performing post-fire assessments. The key insight is that SRC members, despite significant strength degradation at high temperatures, maintain a level of deformation resistance and residual load-bearing capacity that makes them suitable for use in fire-exposed environments. For the piping and structural engineering community, this research reinforces the importance of composite design strategies and the critical role of steel confinement in enhancing the overall structural performance under extreme thermal conditions. Engineers should always consider the full thermal history and loading conditions when evaluating post-fire structural integrity.